docs: work on README

src/AnthropicClient/Models/Usage.cs

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+using System.Text.Json.Serialization;
+
+namespace AnthropicClient.Models;
+
+/// <summary>
+/// Represents the usage of a response.
+/// </summary>
+public class Usage
+{
+  /// <summary>
+  /// Gets the number of input tokens used.
+  /// </summary>
+  [JsonPropertyName("input_tokens")]
+  public int InputTokens { get; init; }
+
+  /// <summary>
+  /// Gets the number of output tokens used.
+  /// </summary>
+  [JsonPropertyName("output_tokens")]
+  public int OutputTokens { get; init; }
+}